In portable radio devices, mobile communication devices in particular, the antenna is preferably located within the covers of the device for user convenience. An internal antenna of a small-sized device is usually a planar type antenna because in that case it is easiest to achieve satisfactory electrical characteristics for the antenna. A planar antenna includes a radiating plane and a ground plane parallel thereto. To make impedance matching easier, the radiating plane and the ground plane are usually interconnected at a suitable point through a short-circuit conductor, resulting in a planar inverted F antenna (PIFA).
FIG. 1 shows a known PIFA type internal multiband antenna. Depicted in the figure there is a circuit board 101 of a radio device, which circuit board has a conductive upper surface. This conductive surface serves as a ground plane 110 in the planar antenna. At one end of the circuit board there is the radiating plane 120 of the antenna, which radiating plane lies above the ground plane, supported by a dielectric frame 150. For impedance matching of the antenna there is at the edge of the radiating plane, near a corner thereof, a short-circuit conductor 115, which connects the radiating plane to the ground plane, and the antenna feed conductor 116. For the feed conductor there is a lead-through, isolated from the ground, to an antenna port on the lower surface of the circuit board 101. The radiating plane has a slot 129 in it, beginning from the edge of the plane, near the short-circuit conductor 115, and extending to the inner region of the plane, near the opposite edge. The slot 129 divides the radiating plane into two branches 121, 122 of clearly different lengths, viewed from the short-circuit point of the radiating plane. The PIFA thus has at least two separate resonating frequencies and the corresponding operating bands.
A disadvantage of the structure shown in FIG. 1 is that when trying to achieve a very small device, the space required by the radiating plane within the device may be too big. In principle this disadvantage could be avoided if the radiating plane were fabricated as part of the cover of the device. This, however, would restrict the design of the radiating element and thus make it more difficult to achieve the electrical characteristics desired.
In the prior art, antenna structures are known which include a surface radiator fed by a primary radiator. FIG. 2 shows an example of such a structure. A surface radiator 230 is attached onto the inner surface of the cover 250 of a device. The structure further includes a printed circuit board 202 parallel to the surface radiator, and a strip-like feed conductor 216 of the antenna on that side of the circuit board which is visible in FIG. 2. On the opposite side of the circuit board 202, i.e. on the side facing the surface radiator, there is a conductive plane 210 with a slot-like non-conductive area 220. The center conductor of the feed line 205 is connected to the conductive strip 216 and the sheath to the conductive plane 210 which is thus connected to the signal ground. The antenna is matched by choosing appropriate dimensions for the circuit board 202 with its conductive parts. Moreover, dimensions of the structure are chosen such that the slot 220 resonates in the operating band and emits energy to the surface radiator 230. As the surface radiator, in turn, resonates, it emits radio-frequency energy into its surroundings.
Antennas like the one depicted in FIG. 2 are used in some mobile network base stations, for example. It is conceivable that such an antenna be applied in mobile stations as well. An advantage of such a structure would be that the antenna could be matched without needing to shape the radiator proper. However, little or no space would be saved compared to the structure shown in FIG. 1. An additional disadvantage would be that such an antenna structure would have only one operating band.